Microelectronic workpieces typically include a substrate that has a plurality of components, such as memory cells, that are interconnected with conductive lines and other features. The conductive lines can be formed by forming trenches or other recesses in the workpiece and then depositing a conductive material or other compound in the trenches. The overburden of the conductive materials, which is the portion of the conductive material above the trenches, is then removed to leave discrete lines of conductive material in the trenches.
Electrochemical processes have been used to both deposit and remove metal layers. A typical electrochemical plating process involves depositing a seed layer onto the surface of the workpiece using chemical vapor deposition (CVD), physical vapor deposition (PVD), or other suitable processes. After forming the seed layer, a blanket layer or patterned layer of metal is plated onto the workpiece by applying an appropriate electrical potential between the seed layer and an electrode in the presence of an electroprocessing solution (e.g., an electrolytic solution). For most metals, a cathode is coupled to the seed layer and an anode is immersed in the electroprocessing solution to establish an electrical field between the seed layer and the anode.
Electroprocessing techniques have also been used to remove metal layers from microelectronic workpieces. For example, an anode can be coupled to a metal layer on a workpiece and a cathode can be immersed in an electrolytic solution to remove metal from the surface of the workpiece. In another example, an alternating current can be applied to a conductive layer through an electrolyte to remove portions of the metal. For example, FIG. 1 illustrates a conventional apparatus 60 for removing metal using an alternating current that includes a first electrode 20a and a second electrode 20b coupled to a current source 21. The first electrode 20a is attached directly to a metallic layer 11 on a semiconductor substrate 10, and the second electrode 20b is at least partially immersed in a liquid electrolyte 31 disposed on the surface of the metallic layer 11. The second electrode 20b, for example, can be moved downwardly until it contacts the electrolyte 31. A barrier 22 protects the first electrode 20a from directly contacting the electrolyte 31. The current source 21 applies alternating current to the substrate 10 via the first electrode 20a, the second electrode 20b, and the electrolyte 31 to remove conductive material from the metallic layer 11. The alternating current signal can have a variety of wave forms, such as those disclosed by Frankenthal et al., “Electroetching of Platinum in the Titanium-Platinum-Gold Metallization on Silicon Integrated Circuits” (Bell Laboratories), which is incorporated herein in its entirety by reference.
One drawback with the system shown in FIG. 1 is that it may not be possible to remove material from the conductive layer 11 in the region where the first electrode 20a is attached to the substrate 10 because the barrier 22 prevents the electrolyte 31 from contacting the substrate 10 in this region. Alternatively, if the first electrode 20a is a consumable electrode in contact with the electrolyte, the electrolytic process can degrade the first electrode 20a. Still a further drawback is that the electrolytic process may not uniformly remove material from the substrate 10. For example, discrete areas of residual conductive material having no direct electrical connection to the first electrode 20a (e.g., “islands”) may develop in the conductive layer 11. The discrete areas of residual conductive material can interfere with the formation and/or operation of the conductive lines, and it may be difficult to remove such residual material with the electrolytic process unless the first electrode 20a is repositioned to be coupled to such “islands.”
One approach to mitigate some of the foregoing drawbacks is to attach a plurality of first electrodes 20a around the periphery of the substrate 10 to increase the uniformity with which the conductive material is removed. However, discrete areas of residual conductive material may still remain despite the additional number of electrodes that contact the substrate 10. Another approach is to form the electrodes 20a and 20b from an inert material, such as carbon, so that the barrier 22 is not required. Although this allows more area of the conductive layer 11 to be in contact with the electrolyte 31, inert electrodes may not be as effective as more reactive electrodes (i.e., consumable electrodes) at removing the conductive material. As a result, inert electrodes may still leave residual conductive material on the substrate 10.
FIG. 2 shows another approach to mitigate some of the foregoing drawbacks in which two substrates 10 are partially immersed in a vessel 30 containing the electrolyte 31. The first electrode 20a is attached to one substrate 10 and the second electrode 20b is attached to the other substrate 10. An advantage of this approach is that the electrodes 20a and 20b do not contact the electrolyte. However, islands of conductive material may still remain after the electrolytic process is complete, and it may be difficult to remove conductive material from the points at which the electrodes 20a and 20b are attached to the substrates 10.
International Application PCT/US00/08336 (published as WO/00/59682) discloses an apparatus having a first chamber for applying a conductive material to a semiconductor wafer, and a second chamber for removing conductive material from the semiconductor wafer by electropolishing or chemical-mechanical polishing. The second chamber includes an anode having a paint roller configuration with a cylindrical mechanical pad that contacts both an electrolyte bath and the face of the wafer as the anode and the wafer rotate about perpendicular axes. A cathode, which can include a conductive liquid isolated from the electrolytic bath, is electrically coupled to an edge of the wafer. One drawback with this device is that is, too, can leave islands of residual conductive material on the wafer.
Another existing device is disclosed in U.S. Pat. No. 6,176,992 B1 owned by Nutool of California. This patent discloses an electrochemical-deposition machine having a first electrode contacting a processing side of a wafer, a polishing pad engaging another portion of the processing side of the wafer, and a second electrode underneath the polishing pad. An electrolyte is passed through the polishing pad in contact with the first electrode, the second electrode, and the face of the wafer. During a plating cycle, a direct current is passed through the first and second electrodes to plate metal ions onto the face of the wafer. During a deplating/planarizing cycle, the polarity of the direct current is switched to deplate metal from the wafer while the polishing pad rubs against the face of the wafer.
One concern of the device disclosed in U.S. Pat. No. 6,176,992 is that the deplating/planarizing cycle may remove material much faster at the perimeter of the wafer than at the center of the wafer. More specially, as the deplating/planarizing cycle progresses, the overburden across the wafer becomes very thin such that a significant voltage drop occurs between the perimeter and the center of the wafer. This voltage drop causes more material to deplate from the perimeter of the wafer than from the center of the wafer. Additionally, the direct current can form a passivation layer on the surface of the metal that exacerbates the voltage drop. This phenomena is particularly problematic for processing large wafers (e.g., 300 mm) because the large diameter of these wafers produces a larger voltage drop. Therefore, it would be desirable to more uniformly remove material from the face of the wafer.